Trunk transmission equipment conforming to the SONET/SDH standards is being installed in trunk transmission lines. In this type of equipment, overall unit size has been decreasing because of the use of large-scale ASICs and the reduced size of the optical modules. Further, the demand for higher transmission capacity is driving the need to increase the line accommodating capacity per piece of equipment (shelf). For these reasons, and for purposes of achieving efficient line switching control, termination control of overhead information (especially, APS (Auto Protection Switch) information as switching information) arriving from many different lines is being managed centrally.
In this case, overhead data from a plurality of lines are concentrated and terminated at one overhead data terminating block where APS, DCC (Data Communication Channel), OW (Order Wire), etc. are separated. Of these, DCC, OW, etc. are all supplied to a lower-order processing block for processing. Accordingly, as the number of signal lines between the overhead data terminating block and the lower-order processing block becomes enormous as the number of accommodated lines increases, the number of signal lines must be reduced by multiplexing the signals. Reduction in the number of signal lines is mandatory particularly when the overhead data terminating block is implemented on one ASIC, because there is a limit on the number of pins on the LSI.
On the other hand, as the data phases of the DCC and OW signals contained in the overheads from the different lines are asynchronous between the lines, if they are multiplexed as they are, processing at the receiving end will become complex, resulting in the problem that the amount of circuitry increases.
Furthermore, in the case of DCC, not only the data phase but also the bit rate may differ. In such a case, if signals of the same bit rate are multiplexed together, as many signal lines as the number of bit rates used would become necessary.